Cycle 4: July the 29th to August the 4th

Hi guys:

This cycle we will work on facts that support the Evolution theories.

1. Check this video

2. Read this information and ask for concepts you don´t understand

THE DISCOVERY OF RIBOZYMES

The central role for many proteins in a cell is to catalyze chemical reactions that are essential for the cell's survival.  These proteins are known as enzymes.  Until relatively recently, it was thought that proteins were the only biological molecules capable of catalysis.  In the early 1980s, however, research groups led by Sidney Altman and Thomas Cech independently found that RNAs can also act as catalysts for chemical reactions. This class of catalytic RNAs are known as ribozymes, and the finding earned Altman and Cech the 1989 Nobel Prize in Chemistry.

The ribozyme isolated by the Cech group, known as the Tetrahymena ribozyme, is shown in the box to the right. It acts to cut a longer strand of RNA into two smaller segments.

THE RNA WORLD HYPOTHESIS

The discovery of ribozymes supported a hypothesis, known as the RNA World Hypothesis, that earlier forms of life may have relied solely on RNA to store genetic information and to catalyze chemical reactions.  This hypothesis was proposed independently by Carl Woese, Francis Crick and Leslie Orgel in the 1960s -- decades before the discovery of ribozymes -- and soon after the double-helical structure of DNA was determined. According to the RNA World Hypothesis, life later evolved to use DNA and proteins due to RNA's relative instability and poorer catalytic properties, and gradually, ribozymes became increasingly phased out.

The ribosome, a large molecular machine that drives protein synthesis, is a ribozyme. Roll over to compare the ribosome structure with and without proteins. Proteins are shown in green, and RNA is shown in blue and white. (PDB #2HGR for the 30S subunit and #2HGU for the 50S subunit).

Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme.  Although the ribosome is made up of both RNA and protein components, structural and biochemical analyses revealed that the mechanisms central for translation (the process of assembling a peptide chain based on a RNA sequence) is catalyzed by RNA, not protein. This suggests that the use of RNA by early lifeforms to carry out chemical reactions preceded the use of proteins.

Rollover the image of the ribosome on the left to compare the structure with and without its protein components.

Next: Explore the role of ribozymes in protocells.

To learn more about ribozymes and the RNA World Hypothesis, check out Links to Learn More.

To download any of the illustrations or animations seen here, visit the Resources for Educators section.

Taken from: http://exploringorigins.org/ribozymes.html

How did life originate?

Living things (even ancient organisms like bacteria) are enormously complex. However, all this complexity did not leap fully-formed from the primordial soup. Instead life almost certainly originated in a series of small steps, each building upon the complexity that evolved previously:

1. Simple organic molecules were formed.
   Simple organic molecules, similar to the nucleotide shown below, are the building blocks of life and must have been involved in its origin. Experiments suggest that organic molecules could have been synthesized in the atmosphere of early Earth and rained down into the oceans. RNA and DNAmolecules — the genetic material for all life — are just long chains of simple nucleotides.
   
2. Replicating molecules evolved and began to undergo natural selection.
   All living things reproduce, copying their genetic material and passing it on to their offspring. Thus, the ability to copy the molecules that encode genetic information is a key step in the origin of life — without it, life could not exist. This ability probably first evolved in the form of an RNA self-replicator — an RNA molecule that could copy itself.
   
   Many biologists hypothesize that this step led to an "RNA world" in which RNA did many jobs, storing genetic information, copying itself, and performing basic metabolic functions. Today, these jobs are performed by many different sorts of molecules (DNA, RNA, and proteins, mostly), but in the RNA world, RNA did it all.Self-replication opened the door for natural selection. Once a self-replicating molecule formed, some variants of these early replicators would have done a better job of copying themselves than others, producing more "offspring." These super-replicators would have become more common — that is, until one of them was accidentally built in a way that allowed it to be a super-super-replicator — and then, that variant would take over. Through this process of continuous natural selection, small changes in replicating molecules eventually accumulated until a stable, efficient replicating system evolved.
   
3. Replicating molecules became enclosed within a cell membrane.
   The evolution of a membrane surrounding the genetic material provided two huge advantages: the products of the genetic material could be kept close by and the internal environment of this proto-cell could be different than the external environment. Cell membranes must have been so advantageous that these encased replicators quickly out-competed "naked" replicators. This breakthrough would have given rise to an organism much like a modern bacterium.

   Cell membranes enclose the genetic material.

4. Some cells began to evolve modern metabolic processes and out-competed those with older forms of metabolism.
   Up until this point, life had probably relied on RNA for most jobs (as described in Step 2 above). But everything changed when some cell or group of cells evolved to use different types of molecules for different functions: DNA (which is more stable than RNA) became the genetic material, proteins (which are often more efficient promoters of chemical reactions than RNA) became responsible for basic metabolic reactions in the cell, and RNA was demoted to the role of messenger, carrying information from the DNA to protein-building centers in the cell. Cells incorporating these innovations would have easily out-competed "old-fashioned" cells with RNA-based metabolisms, hailing the end of the RNA world.
   
5. Multicellularity evolved.
   As early as two billion years ago, some cells stopped going their separate ways after replicating and evolved specialized functions. They gave rise to Earth's first lineage of multicellular organisms, such as the 1.2 billion year old fossilized red algae in the photo below.

   These fossils of Bangiomorpha pubescens are 1.2 billion years old. Toward the lower end of the fossil on the left there are cells differentiated for attaching to a substrate. If you look closely at the upper part of the fossil on the right, you can see longitudinal division that has divided disc-shaped cells into a number of radially arranged wedge-shaped cells, as we would see in a modern bangiophyte red alga.

Taken from: http://evolution.berkeley.edu/evosite/evo101/IIE2bDetailsoforigin.shtml

3. Print the worksheet, 1 per pair. 

https://drive.google.com/file/d/0B-EoRseIFS9BdXlLU2xYd2FiMHM/edit?usp=sharing

See you on class!!!